A magnetic planetary gear speed reduction device is conventionally known (Patent Reference 1, for example). A planetary gear speed reduction device disclosed in Patent Reference 1 is equipped with a sun gear which is a magnetic gear and four planetary gears which are also magnetic gears. The sun gear and the planetary gears are formed cylindrically; the four planetary gears are arranged to surround the sun gear from the outer circumferential side. On the outer circumference side of the four planetary gears, a cylindrical housing yoke is arranged. In this planetary gear speed reduction device, the outside diameter of the sun gear is smaller than the outside diameter of the planetary gears.
Further, in the planetary gear speed reduction device disclosed in Patent Reference 1, the sun gear is secured on the outer circumferential surface of an input shaft and also secured to the outer circumferential surface of rotary shafts. The rotary shafts to which the planetary gears are secured are rotatably held by a planetary carrier.